Many mobile communications systems, for example implemented using the radio access standard known as LTE (Long Term Evolution), make use of the concept referred to as MIMO (multiple input, multiple output), i.e., multiple parallel wireless communication signal streams originating from one (distributed) location and intended for another (distributed) location with all signal streams using the same time and frequency resource.
For example, for a desired performance, transmitting antennas for the individual signals are suitably arranged to inject signal power into the propagation environment in such a way as to create an effective propagation channel with high rank when the signals are detected by similarly suitably arranged antennas at the receiving end of a multi-stream wireless communication link.
In the following, the two ends of a communication link will be denoted “access point” and “terminal”, these two representing for example a macro base station and laptop computer, respectively, exchanging information in a mobile communications system.
A propagation channel has a certain rank, where the rank corresponds to a measure of the number of independent routes a signal may propagate for a given number of antenna ports; in other words the rank is the number of degrees of freedom which is granted by a certain arrangement of antenna functions and the environment where the signal propagates i.e., the wave propagation channel. How antenna functions are arranged in the propagation environment affects the rank.
A high rank propagation channel, i.e., a channel with a rank close to the number of available independent signals, can be achieved by arranging antennas in such a way that variations in amplitude and phase of the individual transferred signals become decorrelated. Spatially separated antennas as well as orthogonally polarized antennas are two common means for achieving decorrelation. Which one of these two arrangements that is the preferred one depends on the specific propagation environment and the properties of the access point and the terminal.
In some MIMO implementations, for example LTE systems, a concept called pre-coding is available. Pre-coding involves a coherent transmission of the same signal from more than one antenna. This results in beam-forming or polarization forming, depending on the properties of the antennas used for transmitting the signal and also depending on the propagation environment. For co-polarized antennas covering essentially the same angular space in an idealized free-space environment, the transmitted signals will add coherently, resulting in beams or, more generally, an interferometer-like electromagnetic field behavior, with in- and out-of-phase signal addition for different directions in space, which depends on the antenna separation distance. Similarly, for antennas with different, for example orthogonal, polarizations, different polarization states will result for different directions in space.
Real propagation environments will cause a less distinct division between beam-forming and polarization forming. However, for many antenna installations, for example when the transmitting antennas are placed and oriented in an access point to radiate on downlink into an effective free-space environment, i.e., an environment without any significant obstacles present between the antenna and the so-called Fraunhofer region, farfield patterns can and will develop before the effects of scattering and multi-path propagation set in. This is of course works reciprocally on uplink.
In previously known solutions, the same antennas or antenna functions are used for transmission and reception. For a TDD (Time Division Duplex) system, the transmitted and received signals are separated in time, and for an FDD (Frequency Division Duplex) system, the transmitted and received signals are separated in frequency. Alternatively, independent radio chains are used for transmission or reception for those antennas that are used only for uplink or downlink. Some implementations of switched antenna systems used for wireless LAN provide an example of the latter whereas conventional base stations in all present standards are an example of the former.
The first existing solution, a system using the same antenna function for downlink and uplink, as manifested by the radiation pattern properties, cannot provide simultaneous optimized performance of both links, if the downlink and uplink system performance have different optima with respect to the radiation pattern. An example is that in certain scenarios beam-forming with closely spaced co-polarized antennas can be optimal for downlink while receiver diversity with dual-polarized antennas can be optimal for uplink.
The second existing solution, a system using independent radio chains or, at the very least, separate feeder lines, will require more feeder cables than the number of parallel signal streams. For example, a 2×2 MIMO system with two co-polarized antennas used for beam-forming on downlink, and two orthogonally polarized antennas used for diversity reception on uplink, will require three feeder cables.
There is thus a need for a node in a wireless communication system where the antenna functions can be made to differ for downlink and uplink in an efficient manner.